There is this note on page 18 of the specification text
Every ‘page’ in a multi-page TIFF, either classic TIFF or BigTIFF, is represented by exactly one IFD and one BIFD
respectively. Please note that BIFD and IFD can not be used together in the same file.
This means that BigTIFF must contain only BIFDs and IFDs are not allowed. This could be clarified by adding a new item into Requirement 3 and a corresponding test into Abstract Test A.3.
If I understand right, the Permission 2, item B deals only with the datatype of tags, and even the tag is allowed to be of datatype IFD, the target that it is points to must be BIFD.
Tags that point to other IFDs, such as the SubIFDs tag (see TIFF PageMaker version 6), are allowed to
have the datatype IFD8. The datatypes IFD (and the not recommendable LONG) are still valid
There is this note on page 18 of the specification text
This means that BigTIFF must contain only BIFDs and IFDs are not allowed. This could be clarified by adding a new item into Requirement 3 and a corresponding test into Abstract Test A.3.
If I understand right, the Permission 2, item B deals only with the datatype of tags, and even the tag is allowed to be of datatype IFD, the target that it is points to must be BIFD.